I agree with Helio, I have never yet found a good
ticket price on-line. RL Travel Agents are less expensive,
particularly the discounters. On-line quotes can be up to
US $1,000 - $2,000 more expensive than the "real" price.

The only luck I've ever had with an on-line booking was with
Ryanair, but then that's directly with Ryanair, not through
an internet agent.